Fast-charging, high-capacity energy storage devices, such as supercapacitors and lithium (Li) ion batteries, are used in a growing number of applications, including portable electronics, medical devices, transportation, grid-connected large energy storage, renewable energy storage, and uninterruptible power supplies (UPS). In modern rechargeable energy storage devices, the current collector is made of an electric conductor. Examples of materials for the positive current collector (the cathode) include aluminum, stainless steel, and nickel. Examples of materials for the negative current collector (the anode) include copper (Cu), stainless steel, and nickel (Ni). Such collectors can be in the form of a foil, a film, or a thin plate, having a thickness that generally ranges from about 6 to 50 μm.
A typical lithium ion battery consists of a carbon anode and a lithium metal oxide or phosphate cathode separated by an electrolyte liquid consisting of a lithium salt such as LiPF5, LiBF4, or LiClO4 in an organic solvent such as ethylene carbonate, or by a solid polymer electrolyte, such as polyethylene oxide, complexed with lithium salts and/or filled with liquid electrolytes. The cathode material is typically selected from lithium transition metal oxides, such as LiMn2O4, LiCoO2, LiNiO2, or combinations of Ni, Li, Mn, and Co oxides and includes electroconductive particles, such as carbon or graphite, and binder material. The cathode material is considered to be a lithium-intercalation compound, in which the quantity of conductive material is in the range from about 0.1% to about 15% by weight. The cathode material may be applied to a conductive sheet electrode as a paste and compacted between hot rollers, or sprayed on as a solution or slurry, and the resulting substrate dried to remove the liquid carrier.
Graphite is frequently used as the anode material and can be in the form of a lithium-intercalation meso-carbon micro bead (MCMB) powder made up of MCMBs having a diameter of approximately 10 μm. The lithium-intercalation MCMB powder is dispersed in a polymeric binder matrix. The polymers for the binder matrix are made of thermoplastic polymers including polymers with rubber elasticity. The polymeric binder serves to bind together the MCMB material powders to preclude crack formation and prevent disintegration of the MCMB powder on the surface of the current collector. The quantity of polymeric binder is in the range of about 0.5% to about 15% by weight. The polymer/MCMB mixture may be applied as a paste and compacted between hot rollers, or in a liquid solution, and the resulting substrate dried to remove the solvent.
Some Li-ion batteries use separators made from microporous polyolefin polymer, such as polyethylene foam, which are applied in a separate manufacturing step. The separator is generally filled with a liquid electrolyte, as described above, to form the finished battery.
As the use of thin-film Li-ion batteries continues to grow, there is an ongoing need for thin-film Li-ion batteries that are smaller, lighter, and can be more cost effectively manufactured.